1. Field of the Invention
The present invention relates to a microwave and luminous probe which detects the piston position in a combustion chamber of an internal combustion engine by employing a microwave and the light emitted by combustion in the combustion chamber by means of a photosensor.
2. Description of the Prior Art
In a conventional microwave and luminous probe of the above-described type, light is linearly propagated through a transmission line to a detector portion, while the course of the microwave is changed by 90.degree., whereby both of them are separated from each other. Since it is necessary to change the course of the microwave by 90.degree. as described above, a structure comprising two waveguide to coaxial adaptors (including end portions of coaxial lines) and a waveguide therebetween has beed used. This structure serves to transmit the microwaves between two different coaxial lines. Since the structure has a cross-sectional width larger than a half of the wavelength of the microwave within the waveguide, it disadvantageously involves an increase in size of the probe and consequently inconvenient handling. Further, it is necessary to give special consideration to the designing of the probe so that it does not interfere with other components of the engine.
Furthermore, the converting portion can transmit microwaves within only a relatively narrow frequency band. On the other hand, the shape of combustion chambers of internal combustion engines differs for each type of engine, and it is necessary to change the wavelength of the microwave used in accordance with the shape of the combustion chamber employed. Accordingly, it is disadvantageously necessary to change the dimensions of the converting portion in accordance with the type of engine.